Having grown up in Jacksonville, I POD a lot of stuff above - St. Simons (beautiful!) and also Big/Little Talbot Islands, Fernandina Beach (much prettier than Daytona or Jacksonville beaches, all of them). St. Augustine - take a night, and do the ghost tours. They are AWESOME. Ripley's is great for most kids - the Castillo de St. Marcos is gorgeous, and great for letting the kids run off some energy and don't miss out on some of the touristy stuff (the old jail, the old schoolhouse). If you go to Scarlet O'Hara's restaurant in the old quarter, the food is amazing, and more ghost-y goodness can be found there (my dad wrote a song for the ghost there, in fact!).
Savannah is great with older kids - lots of history. Incidentally, it's the home of the founder of the Girl Scouts (Juliette Gordon Lowe) and is a really interesting place to visit for that alone - the museum is REALLY cool.
Orlando is overrated, unless you are doing one of the parks - WDW, Sea World, Universal. It's just hot, crowded, and full of tourists. You'll have MUCH more fun elsewhere for the bulk of your time. The parks are great, if pricy, but having spent many, many days down there, it's just really not that much fun unless you are at the parks, or a heavy drinker.
Hope the trip is lovely - it sounds like what we used to do with DD. Such memories!